Self stabilizing electrodes

ABSTRACT

An electrode is described for producing large scale, uniformly distributed glow discharges across a discharge region. The electrode consists of a layer of highly electrically conductive material and a bulk of homogeneous material, the latter having a current-voltage characteristic which is coordinated with the voltage-current characteristic of the discharge region. Any substantial increase in current density through a local area in the discharge region causes a concomitant concentration of current density in the section of the electrode immediately adjacent to the local area; the concentration in the electrode effects a reduction in the electric potential at the electrode surface which in turn reduces the current density through the local area in the discharge. The overall result is a selfcompensating electrode in which the current density is self compensating and uniformly dispersed throughout, and arcing in the discharge is suppressed without external stimulus.

United States Patent [191 Blaszuk July 3, 1973 SELF STABILIZING ELECTRODES [75] Inventor: Paul R. Blaszuk, Marlborough, ABSTRACT Conn An electrode is described for producing large scale, [73 Assignee; United Ai ft Corporation, uniformly distributed glow discharges across a disl-lartford, Conn. charge region. The electrode consists of a layer of highly electrically conductive material and a bulk of [22] F'led: Sept 1971 homogeneous material, the latter having a current- [21] App], N 178,961 voltage characteristic which is coordinated with the voltage-current characteristic of the discharge region. Any substantial increase in current density through a [52] US. Cl 313/217, 313/218, 3 13/31 1 local area in the discharge region causes a concomitam [51] hit. Cl. H01] 17/06 concentration of current density in the Section of the [58] Fleld of Search 313/217, 218, 311 electrode i ly adjacent to the local area; the concentration in the electrode effects a reduction in [56] References C'ted the electric potential at the electrode surface which in UNITED STATES TE T turn reduces the current density through the local area 3,482,138 12 1969 Vollmer 313/218 x n th discharge- The overall result is a self- 3,020,436 2/1962 Ahsmann 313/218 X compensating electrode in which the current density is 3,312,853 4/1967 Mela 313/218 self compensating and uniformly dispersed throughout I Primary ExaminerR0y Lake Assistant Examiner-James B. Mullins Att0rneyAnthony .l. Criso and arcing in the discharge is suppressed without external stimulus.

5 Claims, 5 DrawingvFigures Patented July 3, 1973 3,743,881

2 Sheets-Sheet 1 5% flrrae/z/ar Patented July 3, 1973 v 3,743,881

2 Sheets-Sheet 2 A a: V (m4 7/966) (aaeeawy V (V04 7,965) A V0 SELF STABILIZING ELECTRODES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electrodes and more particularly to electrodes for providing uniformly distributed electric discharges.

2. Description of the Prior Art The discharge of electrical power across media such as a liquid, a gas or vacuum has been known and used repeatedly in various machines. The discharge can assume various forms and widely different characteristics are often desirable, depending upon the particular application involved. In some instances, electrical energy having a high power density is transferred through a discharge medium to create an intense plasma in the form of a highly ionized arc. In a contrastingly different situation, an electrical power having a relatively low power density is transferred through a medium to provide a plasma having an evenly distributed current density containing a small percentage of ions. The techniques and equipment used to produce electric discharges which have characteristics approaching either of the extreme conditions just described vary considerably.

In the operation of an electric discharge molecular gas laser, for example, it is often desirable to produce a plasma discharge having both some characteristics which are typical of a highly ionized discharge and others that are typical of a poorly ionized discharge. More specifically, in a gas laser, an ideal electric discharge conducts a large amount of electrical current in order to allow a high output of laser power while avoiding current densities sufficiently high to cause streamering or arcing since this populates the lower energy levels and reduces the preferred energy level distribution in the laser medium.

Various approaches have been used to increase the amount of electrical power transferred into a discharge medium without incurring arcing or streamering in the plasma. One generally accepted method is to provide a multiplicity of cathode elements so a given amount of power is transferred to a discharge medium from several electrode points thereby limiting the tendency for the electrical current to concentrate adjacent to any one cathode, a condition that approaches a uniform distribution of current throughout the medium. Conceptually, the multiple electrode method is acceptable; however, as a practical matter such systems usually end up with relatively elaborate circuitry requirements which mitigate their usefulness; further, the current density is never truly uniform throughout the medium.

SUMMARY OF THE INVENTION A principal object of the present invention is to provide a simple electrode'which is capable of sustaining an electric discharge which is uniformly dispersed throughout a conducting medium.

The present invention is predicted on my discovery that the tendency for electric current to concentrate at discrete locations on the surface of an electrode can be substantially eliminated by correlating the resistivity of the electrode material to the current density to be passed through the electrode. The invention is further predicted on my discovery of a self-compensating electrode characteristic, namely, the propensity of an electrode, comprised of a homogeneous bulk material having a properly selected electrical resistivity, to undergo a predictable self-compensating change in electrical potential across a local area within the bulk in response to any change in the current density in the local area as an electric current is passed through the electrode; an increase in current density causes in sequence an increase in electrical potential and in turn adecrease in current density.

According to the present invention, an electric discharge having a current uniformly distributed throughout is maintained as a nonarcing, nonstreamering plasma between two electrodes due to the selfcompensating current redistribution characteristic of the electrodes; the resistivity of the electrode material is matched to the voltage current characteristic of the plasma so that for any transient increase in current density in one particular region of the plasma which causes a corresponding increase in current density in the bulk material of the electrode adjacent to the particular region, the increased current density in the bulk material causes a concomitant increase in the electric potential drop across the bulk material involved internal of the electrode, and this in turn decreases the current density at the electrode plasma interface which reduces the current density in the particular plasma region of concern, thereby resulting in quenching of regions of high current density in the plasma. In further accord with the present invention, a first electrode having a substantially flat surface and comprising a layer of high conductivity contact material joined to a homogeneous bulk material of preselected resistivity is spaced apart from a second electrode to form a discharge region therebetween in which a discharge is maintainable by passing a current therethrough; the contact material insures that the current enters the bulk material in a uniformly distributed pattern and any local concentrations of current which occur in the plasma cause both a corresponding current concentration and a lowering of electric potential at the flat surface of the electrode, the latter producing a change in electric potential in the bulk material that reduces the current concentration in the plasma.

One feature of the present invention is the low resistance contact material which forms an integral part of the electrode; this material allows the electric current from the source to be introduced into the bulk material of the electrode in a uniform manner. The electrode is further characterized by the homogenuity of the bulk material which has a preselected resistivity to provide the self-compensating feature of this invention.

Another feature of the present invention is its ability to compensate for current concentrations occurring in the bulk material to provide a uniform electric discharge across the entire discharge region between the electrodes without an external stimulus. This invention is also characterized in that a single cathode having a large surface is capable of providing a large amount of electrical current to an electric discharge without current densities which result in arcing or streamering of the current through the discharge. The present invention is simple in construction and inexpensive to build.

Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of preferred embodiments thereof as illustrated in the accompanying drawmg.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified perspective schematic drawing of a pair of electrodes forming a discharge region therebetween in accordance with the present invention;

FIG. 2 is a simplified perspective drawing of the cathode electrode of FIG. 1 with contour lines of constant current density and constant electric potential;

FIG. 3 is a simplified diagram of the selfcompensating electrode shown in FIG. 2 in a molecular gas laser environment;

FIG. 4 is a graph of a voltage current characteristic for a typical electrode bulk material; and

FIG. 5 is a graph of a voltage current characteristic for a typical plasma discharge maintained with electrode in accordance with the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT The present invention utilizes an electrode construction which is self stabilizing with respect to current path therethrough in maintaining an electric discharge; although the discharge can be across a region of gas, a liquid or a vacuum, the electrode will be discussed herein primarily as used with a gas. The electrode makes possible the transfer of relatively large amounts of electrical power to a gas in a manner which avoids arcing of current along the electrical paths in the discharge formed in a gas. If a substantial electric current concentration should form in the discharge, the current density in the electrode adjacent to the concentration initially tends to increase in order to support the discharge concentration, causing a concomitant change in the electric potential across the electrode. The change in potential causes a reduction in the electric potential at the electrode surface which in turn reduces the current density at the surface, resulting in a more uniform redistribution of current throughout the electrode and in turn the discharge.

A self-stabilizing electrode in accordance with this invention is shown in FIG. 1 as one element of a simplified schematic of a gas discharge apparatus. A cathode electrode, identified by general reference character 10, comprises a layer 12 of high electrical conductivity material and a slab 14 of homogeneous bulk material having a preselected resistivity. The slab has a discharge surface 16 which is substantially flat and extends across the entire electrode. A pair of electrodes comprising the cathode and an anode 18 is connected to a power supply 20 by wires 22, 24 to form an electric discharge circuit having a discharge region 26 that is located between the electrodes.

In the production of a uniformly distributed electric discharge in the region 26, gas (not shown in the drawing) is provided between the electrodes and electric current is provided to the cathode by the power supply 20; the layer introduces this current intothe' slab of bulk material in the form of a uniform current density across the entire interface between the layer and the slab. Under the desired conditions of operation, electrons flow from the cathode to the anode and the current density in the bulk material is essentially uniform throughout as is represented in FIG. 1 by solid lines 28 of constant current density. The described current flow produces an electric field gradient which is represented also in FIG. 1 by dashed lines 30 of uniform electric potential. It should be noted that the lines 30 always remain perpendicular to the lines 28.

The system just described is ideal and under actual operating conditions the current tends to become nonuniformly dispersed in the region 26, and low electrical resistance, high current density electrical paths form between the electrodes. When these paths occur, the lines of constant current density tend to form into a pattern as is shown in FIG. 2; simultaneously, the lines of constant potential which always remain at right angle to the lines of constant current density tend to bend away from the discharge surface 16 as is indicated, and the electric potential at the surface 16 is reduced. As the surface potential is reduced, the electric potential available to maintain the path of higher current density is accordingly reduced, reducing the power transferred into the discharge region at the point of reduced surface potential. The current in the cathode then redistributes itself around the location of concern, the net result being the overall power transferred to the gas remains constant and the current concentrations in the electrode are eliminated by the self-compensating propensities of the electrode material which are preselectedly matched to the voltage current characteristic of the discharge medium.

Generally speaking, the self-compensating electrodes has a higher resistivity when used in cw operation than when a pulsed discharge is being produced. This is due to the fact that the voltages and currents required for pulse mode of operation are much higher than would be for a system running cw and, therefore, a lower electrode resistivity will produce the desired effect on changes in electric potential in response to changes in current density in accordance with the relationship where E electric potential,

p material resistivity, and

J current density.

Determining the resistivity of an electrode in accordance with this invention can be explained in terms of an electric circuit comprised of a power source connected in series with two resistances, one resistance representing the electrode and the other resistance representing the discharge region. The power source has a negative voltage characteristic so that as the current drawn from the source is increased, the potential at which it is provided decreases; the bulk material has a positive voltage current characteristic as is shown typically in FIG. 4 and the plasma region has an essentially negative voltage current characteristic as is shown typically in FIG. 5. A stable electric potential across the discharge region is established by determining a current at which the material in the discharge region will maintain a glow discharge such as is represented by point 1 in FIG. 4. The electrode then selected must have a resistivity such that the current passing through the series circuit is consistent with the voltage current characteristic of the discharge region such as point 1 in FIG. 5. As long as the characteristic for the bulk material has a slope such that a given change in current produces an increase in potential from point 1 to point 2 such as AV 1-2) thereacross which is greater than the decrease in potential AV (1-2) in the plasma, the circuit is stable.

The present invention is shown schematically in a gas laser embodiment in FIG. 3. The cathode 10 and the anode 18 are supported by side wall structure 31 and the discharge region 26 is formed in the laser optical cavity formed by the mirrors 32 and 34. The system which can be static or flowing produces a beam 36 of output laser energy. In operation, the cathode comprises a thin layer 12 of highly conductive material such as silver, gold or copper and a bulk material 14 such as graphite (generally pulsed) or silicon, germanium and gallium arsinide (generally used for cw operation). The anode i8 is any satisfactory electrode material such as copper, and the gas filling the discharge region 26 is any laser gas such as a carbon dioxide nitrogen, helium mixture. The power supply is shown as a source of direct current; although it can be either direct or alternating current and it can be supplied to the discharge region either continuously or in an intermittent fashion. When power is provided, a discharge is established in the region 26 and a population inversion is established in the laser medium in the optical cavity, producing the beam of output energy.

During the operation of the device, the electric power which is being supplied as a uniform current density across the entire surface 16 of the cathode can tend to form streamering or arcing paths between the electrodes l0, 18 resulting in uneven current distributions which destroy the glow discharge. As these paths of relatively high current density form in the discharge, the current density in the cathode bulk becomes unevenly distributed as is shown schematically in FIG. 2. Distortion of the lines of constant current density in the bulk cathode material distorts the lines of equal potential in the region adjacent to the current concentration in the discharge, as is shown in FIG. 2, resulting in a lower electric potential at the surface 16; a reduced potential across an arcing path in the discharge causes the current imposed along the path to decrease. This sequence tends to reduce the current in the high current path in the discharge and the electrode will redistribute the power distribution throughout its bulk with no outside influence.

Although this invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.

Having thus described a typical embodiment of my invention, that which I claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for producing large scale uniformly distributed glow discharges comprising:

an electric discharge chamber;

means for providing electric power to said chamber;

and

a pair of spaced apart electrodes, which are electrically connected to said power means and disposed within said chamber to form a discharge region therebetween comprising:

a flat discharge surface-cathode element formed of a bulk material backed with a thin layer of highly electrically conductive material, the bulk material having a spatially homogeneous cross section and a resistivity (p) characteristic which when subjected to an electric potential (E) produces a current density (J) according to the relationship the resistivity (p) matched to the electrical characteristics of said region so that for a given incremental increase in current density across a local cross sectional area in the discharge which causes a concomitant increase in a current density across a corresponding area in the cathode adjacent to the local area, the drop in electric potential across the discharge is less than the increase in potential across the cathode thereby reducing the current density across both the local area in the discharge and the corresponding area in the cathode; and

an anode element.

2. The invention according to claim 1 wherein the bulk material is substantially graphite.

3. The invention according to claim 1 wherein the bulk material is substantially one of the materials selected from the group consisting of silicon, germanium and gallium arsinide.

4. The method of discharging an electric current uniformly across a discharge region with a self-stabilizing electrode which self compensates for current concentrations therein including the steps of:

providing an electrical circuit having a discharge region, an anode and a cathode which are electrically connected in series to a source of electrical power, the discharge region being formed between the anode and cathode, the cathode being formed of a bulk material backed with a thin layer of highly electrically conductive material having a positive current voltage discharge characteristic so that an increase in current density across any local area therein is accompanied by an increase in the electric potential across the local area;

providing in the discharge region a medium which will sustain an electric discharge, the medium having a negative voltage-current discharge characteristic so that an increase in current density within a local area therein is accompanied by a decrease in electric potential across the local area;

matching the resistivity of the cathode to the voltagecurrent characteristic of the medium so that a given increase in current density in the medium has an associated decrease in voltage drop across the discharge, said decrease being less than the concomitant increase in voltage across the cathode; and

producing a flow of electric current through the circuit with the power source and passing the current through the conductive material and into the bulk material at a substantially uniform current density across the entire bulk material cross section and maintaining a substantially uniform current density throughout the entire bulk material.

5. The method of maintaining a nonarcing, nonstreamering electric discharge across a gaseous medium with a self-stabilizing, self-compensating flat surfaced electrode comprised of bulk material having a preselected resistivity and a highly conductive layer of contact material including the steps of:

establishing a discharge region between an anode and a cathode which are connected in a series circuit to a source of electrical power, the cathode comprised essentially of a bulk material which is spatially homogeneous in cross section and which has a voltage current characteristic that requires an increase in potential in order to increase the current density thereacross;

providing a discharge medium to the discharge retial decrease and increase respectively in the adjagion, the medium having a negative voltage current cent local area in the cathode bulk material; discharge characteristic; providing electric power from the source at a voltage matching the resistivity of the cathode bulk material which is sufficient to overcome the electrical resisto the electrical characteristics of the discharge 5 tance offered by the cathode, the discharge memedium so for an increase in current density across dium and the circuit; and a local area in the discharge medium which causes maintaining a substantially uniform current density a corresponding decrease in the electric potential essentially throughout the entire bulk material. thereacross, the current density and electric poten- 

1. Apparatus for producing large scale uniformly distributed glow discharges comprising: an electric discharge chamber; means for providing electric power to said chamber; and a pair of spaced apart electrodes, which are electrically connected to said power means and disposed within said chamber to form a discharge region therebetween comprising: a flat discharge surface cathode element formed of a bulk material backed with a thin layer of highly electrically conductive material, the bulk material having a spatially homogeneous cross section and a resistivity ( Rho ) characteristic which when subjected to an electric potential (E) produces a current density (J) according to the relationship E Rho J, the resistivity ( Rho ) matched to the electrical characteristics of said region so that for a given incremental increase in current density across a local cross sectional area in the discharge which causes a concomitant increase in a current density across a corresponding area in the cathode adjacent to the local area, the drop in electric potential across the discharge is less than the increase in potential across the cathode thereby reducing the current density across both the local area in the discharge and the corresponding area in the cathode; and an anode element.
 2. The invention according to claim 1 wherein the bulk material is substantially graphite.
 3. The invention according to claim 1 wherein the bulk material is substantially one of the materials selected from the group consisting of silicon, germanium and gallium arsinide.
 4. The method of discharging an electric current uniformly across a discharge region with a self-stabilizing electrode which self compensates for current concentrations therein including the steps of: providing an electrical circuit having a discharge region, an anode and a cathode which are electrically connected in series to a source of electrical power, the discharge region being formed between the anode and cathode, the cathode being formed of a bulk material backed with a thin layer of highly electrically conductive material having a positive current voltage discharge characteristic so that an increase in current density across any local area therein is accompanied by an increase in the electric potential across the local area; providing in the discharge region a medium which will suStain an electric discharge, the medium having a negative voltage-current discharge characteristic so that an increase in current density within a local area therein is accompanied by a decrease in electric potential across the local area; matching the resistivity of the cathode to the voltage-current characteristic of the medium so that a given increase in current density in the medium has an associated decrease in voltage drop across the discharge, said decrease being less than the concomitant increase in voltage across the cathode; and producing a flow of electric current through the circuit with the power source and passing the current through the conductive material and into the bulk material at a substantially uniform current density across the entire bulk material cross section and maintaining a substantially uniform current density throughout the entire bulk material.
 5. The method of maintaining a nonarcing, nonstreamering electric discharge across a gaseous medium with a self-stabilizing, self-compensating flat surfaced electrode comprised of bulk material having a preselected resistivity and a highly conductive layer of contact material including the steps of: establishing a discharge region between an anode and a cathode which are connected in a series circuit to a source of electrical power, the cathode comprised essentially of a bulk material which is spatially homogeneous in cross section and which has a voltage current characteristic that requires an increase in potential in order to increase the current density thereacross; providing a discharge medium to the discharge region, the medium having a negative voltage current discharge characteristic; matching the resistivity of the cathode bulk material to the electrical characteristics of the discharge medium so for an increase in current density across a local area in the discharge medium which causes a corresponding decrease in the electric potential thereacross, the current density and electric potential decrease and increase respectively in the adjacent local area in the cathode bulk material; providing electric power from the source at a voltage which is sufficient to overcome the electrical resistance offered by the cathode, the discharge medium and the circuit; and maintaining a substantially uniform current density essentially throughout the entire bulk material. 